One application of radio frequency (RF) receivers is to monitor the RF spectrum and identify unknown emitters. For example, in the battlefield, it would be advantageous to able to detect the presence of narrow-band push-to-talk radios, indicating the presence of enemy troops nearby. Once detected, the position of the unknown emitter could be estimated and/or the signal could be demodulated to determine the information content.
One prior art method for identifying a narrow-band transmission at an unknown frequency within a much larger frequency band utilizes a traditional heterodyne receiver that mixes a pure tone with the received RF signal to create a new intermediate frequency (IF), usually at a much lower frequency than the original RF signal. The new IF signal is filtered to remove higher frequency components from the mixing process before being amplified and digitized. The IF signal includes the RF sources within a narrow band of frequencies defined by the low-pass filter and the tone in question. To monitor a large band of the RF spectrum, the LO tone is stepped across the RF band of interest allowing the capture of one IF bandwidth worth of spectrum before the LO frequency is incremented and a new capture begins. For a receiver with 50 MHz IF bandwidth, it would take 20 such measurements to cover a 1 GHz span of RF frequencies.
For this strategy to succeed, the unknown signal must continue to broadcast for the period of time needed to sweep the RF spectrum. If the RF signal appears briefly at a frequency not presently being mixed into the IF band and this signal terminates before the appropriate LO sweep frequency occurs, it will not be detected.
Another option for monitoring a wideband portion of the RF spectrum utilizes a ultra fast analog-to-digital converter (ADC) which samples quickly enough to capture the entire RF band of interest without any aliasing. These ultra fast ADCs are typically used in high speed oscilloscopes. Today's oscilloscope ADCs can support sample rates of 80 Giga-samples-per-second (Gs/s) allowing monitoring of RF signals up to 30 GHz. Unfortunately, these ultra fast ADCs have a limited dynamic range, require high power, and have very high cost. The limited dynamic range negatively impacts the ability to detect a low power signal in the presence of higher power background signals. The high cost and high power requirements make such implementations unattractive for devices that are used in the field. In addition, the high volume of data generated by such systems requires significant computing resources to process, which further detracts from such solutions.